Difference between std::result_of and decltype

Question

I have some trouble understanding the need for std::result_of in C++0x. If I understood correctly, result_of is used to obtain the resulting type of invoking a function object with certain types of parameters. For example:

template <typename F, typename Arg>
typename std::result_of<F(Arg)>::type
invoke(F f, Arg a)
{
    return f(a);
}

I don't really see the difference with the following code:

template <typename F, typename Arg>
auto invoke(F f, Arg a) -> decltype(f(a)) //uses the f parameter
{
    return f(a);
}

or

template <typename F, typename Arg>
auto invoke(F f, Arg a) -> decltype(F()(a)); //"constructs" an F
{
    return f(a);
}

The only problem I can see with these two solutions is that we need to either:

• have an instance of the functor to use it in the expression passed to decltype.
• know a defined constructor for the functor.

Am I right in thinking that the only difference between decltype and result_of is that the first one needs an expression whereas the second does not?

Answer 1

result_of was introduced in Boost, and then included in TR1, and finally in C++0x. Therefore result_of has an advantage that is backward-compatible (with a suitable library).

decltype is an entirely new thing in C++0x, does not restrict only to return type of a function, and is a language feature.

---

Anyway, on gcc 4.5, result_of is implemented in terms of decltype:

  template<typename _Signature>
    class result_of;

  template<typename _Functor, typename... _ArgTypes>
    struct result_of<_Functor(_ArgTypes...)>
    {
      typedef
        decltype( std::declval<_Functor>()(std::declval<_ArgTypes>()...) )
        type;
    };